Fluoride ions play a critical part in the prevention of caries. Thus, inter alia, fluoride ions are added to drinking water in order to counter the spread of caries.
Another way of using fluoride ions for combating caries is the use of toothpaste which contains fluoride ions (see "Oral Hygiene Products and Practice" by Morton Pader in "Cosmetic Science and Technology Series", vol. 6, pages 383 et seq. (1988), Marcel Dekker, Inc.). For this purpose, it is known that fluoride ions may be added to toothpastes in the form of NaF, MFP, aminofluoride, tin fluoride, etc. In addition, these toothpastes may contain precipitated silicas which have an abrasive and/or thickening effect (WO 93/24103). The availability of fluoride ions in the oral cavity, however, is substantially restricted to the short time during which the teeth are cleaned.
It is known that hydrofluoric acid can be added to an aqueous slurry of silica gel or xerogel. This slurry is then mixed with the components for preparing a toothpaste, wherein, inter alia, calcium chloride is also added (DE-A 24 16 742). Treatment with hydrofluoric acid causes Si--F bonds to be produced on the silica gel which means that cations, for example calcium ions, are adsorbed less by the silica gel. The formation of fluorine anions and their controlled release in a toothpaste is not described in the document DE-A 24 16 742.
It is known that pyrogenic silicon dioxide which contains 1 to 5% of adsorbed fluoride can be incorporated into toothpaste. This toothpaste always contains 800 to 1200 ppm of free fluoride which is available for appropriate therapeutic treatment (U.S. Pat. No. 4,172,121). The pyrogenic silicon dioxide in this toothpaste, in addition to providing a medium which contains fluoride ions, also acts as a thickening agent which provides the toothpaste with the requisite pasty consistency.
Pyrogenic silica which contains fluoride is prepared by treating pyrogenic silica with HF gas in accordance with U.S. Pat. No. 4,054,689.
It is known that precipitated silica which contains fluoride can be prepared by introducing sodium fluoride and oxalic acid into an aqueous recipient vessel and then precipitating the precipitated silica by simultaneous addition of sodium silicate solution and sulfuric acid (DE-B 12 93 138).
As a result of adding sodium fluoride and oxalic acid, iron ions are complexed in the precipitation mixture and a precipitated silica with a low concentration of iron is obtained. Adsorption of sodium fluoride on the precipitated silica does not take place.
A critical factor for improving the efficacy of oral hygiene preparations is regarded in particular as maintaining the concentration of active substance in the oral cavity for as long as possible after spontaneous release of the active substance. The objective is to improve retention of the active substance and thus also to produce the required effect with reduced amounts of active substance. Undesirable effects due to high concentrations should largely be avoided in this way.
Taking, as an example, the active substance fluoride, whose anticaries effect has been demonstrated in many clinical and epidemiological studies, it can be shown that the desired protection against caries can be produced even with extremely low concentrations (see Y. Ericson, J. Dent. Res. 59 (DII):2131 (1980); O. Backer Dirks, W. Kunzel and J. P. Carlos, Caries Res. 12 (Suppl. 1):7 (1978); A. r. Volpe, in a textbook of Preventive Dentistry (R. C. Caldwell and R. E. Stallard, eds.), Saunders, Philadelphia, 1997, chap. 12).
Furthermore, there are indications that the frequency of fluoride application, i.e. the presence of fluoride ions at the correct time and in the correct place, in the event of acid attack is of greater importance than the fluoride concentration (see "Oral Hygiene Products and Practice" by Morton Pader in "Cosmetic Science and Technology Series" vol. 6, pages 383 et seq., (1988), Marcel Dekker, Inc.).
These findings led to the demand that research activity be directed towards maximising the time of action of fluoride while simultaneously minimising the fluoride concentration. Also, the toxicological aspects of increasing the fluoride doses or increasing the accumulation of fluoride, e.g. due to fluorinated drinking water, mouthwashes, toothpastes, topical fluoride preparations, fluoride tablets, etc., must not be ignored.
Again, in the case of active anti-plaque substances, it has been shown, by in vitro and in vivo studies, that the key to an improved effect is to be found in improved retention of the active substance. Thus, for example, exceptional plaque inhibition is produced by chlorhexidine which is attributed mainly to its retention in the oral cavity and gradual release from the plaque matrix and mucous membranes. A long-lasting antimicrobial effect was detected in early studies by determining the number of bacteria in saliva after rinsing out the mouth once (see K. Kornmann: Antimicrobial agents, in Loe, H & Kleinmann DV: Dental Plaque control measures and oral hygiene practices, Oxford/Washington, IRL Press, 121-142, 1986; P. Germo, P. Bonesvoll, G. Rolla: Relationship between plaque inhibiting effect and retention of Chlorhexidine in the oral cavity. Archs Oral Biol. 19:1031-1034, 1974; W.R. Roberts, M. Addy: Comparison of the bisguanidine antiseptics, alexine and chlorhexidine: i Effects on plaque accumulation and salivary bacteria. J. Clin. Periodontol. 8:8 213-219, 1981).
Another example is triclosan (2,4,4'trichloro-2'-hydroxydiphenyl ether), which has a distinct plaque-inhibiting effect only when combined with a copolymer (polyvinylmethylether/maleic acid) which improves the retention of triclosan (see American Journal of Dentistry, vol. 2, Special Issue, September, 1989: Report on the use of Triclosan/Copolymer Dentifrices in the Control of Plaque and Gingivitis).
It is known that silicas, as highly disperse substances, are able to be stored in fissures, micro-cracks and tubuli in the surface of teeth. The blocking of uncovered dentine tubuli in this way by silica leads to a desensitising effect (see M. Addy, P. Mostafa and R. Newcombe, Dentine Hypersensitivity: A Comparison of five toothpastes used during a six-week treatment period; British Dent. J. 163, 45-50, 1987).
There is, therefore, a need to provide a precipitated silica with the ability to carry active substances as oral hygiene agents, which satisfies all the requirements placed on this type of agent with regard to tolerance, abrasiveness, rheology, sensory and optical properties, and is also able to fix active substances in the oral cavity and then to release them in a controlled manner over a relatively long period of time.